The function of a mammal's circulatory system—its heart, lungs, blood vessels and red blood cells—is to provide oxygen and nutrients to every cell. The heart's role is to pump oxygenated red blood cells to the tissues, to receive deoxygenated blood from the tissues, and to pump deoxygenated blood to the lungs where it can again take up oxygen. Heart failure can be viewed as the failure to fulfill this role.
Heart failure affects more than 2 million Americans, and is a major cause of illness, hospitalization, and death around the world. Currently available therapies includes medications such as digoxin and angiotensin converting enzyme inhibitors, but these have had limited impact on morbidity and mortality. Left ventricular assist devices show promise, but remain experimental. Cardiac transplantation is limited by a shortage of available hearts, and the need for permanent immunosuppression. Thus, improved therapies for heart failure are needed.
To develop improved therapies for heart failure, a more complete understanding of the heart's normal operation is needed. With this more complete understanding, specific aspects of heart function can be targeted for pharmacologic therapy, gene therapy, and other novel therapeutic approaches.
Many basic facts about heart function are known. The heart is largely made up of cardiac muscle, or myocardium. The myocardium mediates the heart's pumping function by automatically contracting and relaxing in a cyclical manner. The contraction drives the blood forward, while the relaxation phase creates negative pressure that helps the heart to fill with blood. This alternation of myocardial contraction and relaxation is termed the cardiac cycle.
In pumping blood, the vertebrate heart takes advantage of the increased efficiency of wringing compared to compression. Just as both hands rotate in opposite directions during the squeeze, the base of the heart rotates in the clockwise direction as the apex rotates counter-clockwise. An advantage of these mechanics is the reduction of chamber volume and consequential decrease in wall stress. The orientations of many cardiac muscle bundles facilitate the wringing as well as compressing forces.
The molecular motor that drives contraction of cardiac muscle is myosin. The role of myosin is to transduce chemical energy into movement by hydrolyzing the high-energy phosphodiester bond of ATP.
Myosin is a large protein made up of three subunits, the myosin heavy chain and two myosin light chains termed essential light chain (ELC) and regulatory light chain (RLC). The myosin heavy chain is an elongated molecule with a filamentous tail and a globular head. A “neck” region lies between the tail and head. The tails self-assemble into filaments, with the myosin head extending outward from the filament. These myosin-containing filaments are termed thick filaments. They interact with thin filaments, which contain actin polymers. The actin polymers activate the ATPase activity found in the myosin head. Movement is generated when the myosin heads: (1) bind to actin filaments; (2) hydrolyze ATP, thereby generating a lever like motion at the myosin neck; and (3) detach from sites on the actin-containing thin filament. The constant repetition of this cycle pushes the thin filament past the thick filaments, thereby generating differential motion. Multiplied over millions of highly organized cardiac cells, the result is a highly coordinated cycle of contraction and relaxation.
The trigger for cardiac contraction is a transient rise in the intracellular level of calcium. The actin-containing thin filament binds an additional protein complex called troponin. In the absence of calcium, troponin interferes with the actin-myosin interaction. However, the troponin complex contains a high-affinity calcium binding protein which binds calcium, thereby triggering a movement of the complex which allows actin and myosin to interact productively. Cardiomyocytes contain intracellular calcium stores that rapidly release calcium and take it back, thereby promoting the cycle of contraction and relaxation.
The neck region of the myosin heavy chain is supported by the two myosin light chains. The precise role of these myosin light chains in cardiac muscle has remained elusive. In smooth muscle (found in blood vessels and internal organs, for example) the RLC plays a critical regulatory role: for contraction to proceed, the RLC must be phosphorylated by a calcium-activated enzyme called myosin light chain kinase (MLCK). In the absence of MLCK-mediated RLC phosphorylation, smooth muscle myosin ATPase activity is not activated, and the muscle remains relaxed.
In stark contrast to smooth muscle, cardiac RLC phosphorylation has little effect on myosin ATPase activity. A modest increase in sensitivity to calcium has been described in isolated, chemically “skinned” (i.e., outer membranes removed) fibers in vitro, but this observation is of doubtful in vivo significance. Nevertheless, a phosphorylatable serine homologous to smooth muscle RLC has been preserved throughout evolution, and the reasons for this conservation have remained a mystery.
Further study of a possible role for cardiac RLC phosphorylation has been significantly hampered by the lack of sequence information about the cardiac form of MLCK. What is needed is the complete cDNA sequence of cardiac MLCK in humans and other mammalian species, as well as the deduced amino acid sequence and genomic sequence.
Indirect flight muscle (IFM) of insects has the same basic contractile apparatus as mammalian cardiac muscle: a myosin based thick filament comprised of myosin heavy and light chains; and an actin-containing thin filament activated by calcium binding to troponin. However, IFM must contract and relax 150 times per second during flight. It would be energetically wasteful to regulate this extraordinarily rapid cycle exclusively through release and reuptake of calcium from intracellular stores. Thus, IFM has evolved to accentuate and exploit a property of muscle contraction termed stretch activation.
The stretch activation response of IFM manifests itself as a “delayed tension” when an activated muscle fiber is subjected to a quick stretch. When tension is monitored as a function of time (for example, by attaching an isolated muscle to a sensitive force transducer), and IFM is quickly stretched, an immediate increase in tension is observed which rapidly decays. This immediate tension increase is mediated by elastic recoil. In IFM, there is a second, delayed rise in tension which is defined as stretch activation. This response has been shown to be a critical component of IFM function, since it contributes substantially to oscillatory power output. Drosophila mutants lacking stretch activation have no ability to fly.
The role of stretch activation can be likened to pushing a child on a swing: when a swing is at the rear of its arc, it has zero velocity and is about to be pulled forward by gravity. A properly timed push is a very efficient way to enhance the forward swinging force. In IFM, stretch activation corresponds to the push.
Stretch activation is intimately related to another important property of IFM, namely resonant frequency. As in the swing metaphor, the swing arc has a predictable frequency, and will return to the pushing individual at a particular time. This predictable frequency is the swing's resonant frequency. The individual must time the push to the resonant frequency. Such precise timing will maximally enhance the swinging motion's amplitude with the least amount of effort. An improperly timed push will not enhance the amplitude, and may in fact work against the swinging motion. Similarly, the resonant frequency of stretch activation in IFM must be precisely matched to the cycle of muscle contraction and relaxation.
Several mutations in human cardiac ELCs and RLCs are associated with an unusual inherited disease of cardiac muscle (cardiomyopathy) termed mid-cavitary ventricular hypertrophy (MCVH; Poetter et al., Nature Genetics 13: 63-69, 1996). In its fully developed form, MCVH is characterized by massive overgrowth or hypertrophy largely confined to the center of the left ventricle—the papillary muscles, and adjacent interventricular septum and left ventricular free walls. The physiologic basis for this unusual, regionally confined hypertrophy is unknown. Interestingly, however, when a mutant human cardiac ELC is expressed in transgenic mice, the mice develop regional hypertrophy indistinguishable from human MCVH. Papillary muscles removed from the hearts of these transgenic mice show altered stretch activation, even before the hypertrophy develops (Vermuri et al., PNAS 96: 1048-1053, 1999). The alteration included a significantly increased resonant frequency.
It would be helpful to determine whether stretch activation has a significant role in mammalian cardiac muscle, and to develop new therapies for heart disease based on modulation of stretch activation. Improved and more comprehensive methods of identifying individuals at risk of developing cardiac dysfunctions, such as cardiomyopathy, would also be beneficial.